Receivers for optical signals are useful in a wide variety of applications including optical communication systems. As presently contemplated, such systems typically have a light source and receiver optically coupled to each other by means of an optical fiber pathway. Information is usually transmitted by varying the intensity of optical radiation and the rate of variation is referred to as the bit rate. An optical receiver detects an incoming optical signal and converts it to an electrical signal.
The optical receiver should have a high sensitivity. The optical receiver should also have a large dynamic range. Additionally, typical optical communications systems operate within a wide range of bit rates. A common approach for achieving medium sensitivity and wide bandwidth for an optical receiver is to use a transimpedance amplifier. (See, e.g., Ogawa, "Considerations For Optical Receiver Design" IEEE Journal on Selected Areas in Communications", VOL SAC-1, No. 3, Apr. 1983, pp. 524-532). This type of receiver comprises an amplifier having non-inverting and inverting inputs and an output. The non-inverting input of the amplifier is grounded. A signal photodetector for receiving the incoming optical signal is connected to the inverting input. A resistor provides electrical feedback between the output and the inverting input to balance the current produced by the signal photodetector. With this configuration, the feedback resistor must be large to produce high gain and sensitivity.
Such a large resistor causes several problems. In particular, the dynamic range of the receiver is limited because large amplifier output voltage swings are needed for an appreciable signal current flow through the feedback resistor. Additionally, the bandwidth is limited by input capacitance combining with parasitic feedback resistor capacitance producing a large RC time constant.
A receiver utilizing optical feedback between the amplifier output and the amplifier input eliminates many of the problems associated with the feedback resistor in a transimpedance amplifier while providing the necessary current feedback to achieve wide dynamic range and wide bandwidth. Thus, the feedback resistor is replaced by an optical feedback path. To achieve the optical feedback path, a light source such as an LED is connected to the output of the amplifier. Connected to the non-inverting input of the amplifier is a feedback photodetector illustratively in the form of a P-I-N photodiode. In addition there is an optical path between the light source and feedback photodetector. During receiver operation, negative feedback as a result of optical transmission between the light source at the amplifier output and the feedback photodetector insures that the optical level at the feedback detector is large enough so that the photo-current in the feedback detector balances the photo-current in the signal detector.
The use of optical feedback has a number of significant advantages. The optical feedback coupling eliminates the feedback resistor as a noise source and the parasitic capacitance as a bandwidth limitation. This results in a higher signal-to-noise ratio, and thus a more sensitive device for optical communications. It also insures that the amplifier output voltage swing is small resulting in a wide dynamic range.
Thus, the use of optical feedback in an optical receiver provides significant advantages over the use of a feedback resistor.
Presently, receivers using optical feedback are implemented with discrete light emitting and photodetecting components in the feedback path (see, e.g., U.S. Pat. No. 4,744,105; U.S. Pat. No. 3,955,149; U.S. Pat. No. 4,284,960; U.S. Pat. No. 4,625,105: U.S. Pat. No. 3,611,173; UK Patent Application 2,030,020A and German Patent Document 2, 218,431).
In contrast to the use of discrete components to implement a light source and photodetector to provide a feedback path for a receiver, it is an object of the present invention to provide a single integrated package including a light emitting device and a detector for use in providing an optical feedback path for a receiver. It is a further object of the invention to provide a single integrated package wherein a light emitting diode and photodetector are bonded to a common submount of a header and wherein the optical medium for coupling light from the LED to the photodetector is a reflective surface mounted above the submount.